‘Right,’ Alleyn said. ‘Now the murderer was faced with evidence of identity. One imagines the trout glistening with a clear spiked heel mark showing on its hide. It wouldn’t do to throw it into the stream or the willow grove and run away. There lay the Colonel with his hands smelling of fish and pieces of cut grass all round him. For all his murderer knew, there might have been a witness to the catch. This, of course, wouldn’t matter as long as the murderer’s identity was unsuspected. But there is a panic sequel to most crimes of violence and it is under its pressure that the fatal touch of over-cleverness usually appears. I believe that while the killer stood there, fighting down terror, the memory of the Old ’Un lying on Bottom Bridge, arose. Hadn’t Danberry-Phinn and the Colonel quarrelled loudly, repeatedly and vociferously – quarrelled that very afternoon over the Old ’Un? Why not replace the Colonel’s catch with the fruits of Mr Phinn’s poaching tactics and drag, not a red-herring, but a whacking great trout across the trail? Would that not draw attention towards the known enemy and away from the secret one? So there was a final trip in the punt. The Colonel’s trout was removed and the Old ’Un substituted. It was at this juncture that Fate, in the person of Mrs Thomasina Twitchett appeared to come to the murderer’s aid.’

‘For God’s sake,’ George Lacklander shouted, ‘stop talking …’ He half-formed an extremely raw epithet, broke off and muttered something indistinguishable.

‘Who are you talking about, Rory?’ Lady Lacklander demanded. ‘Mrs Who?’

‘Mr Phinn’s cat. You will remember Mrs Cartarette told us that in Bottom Meadow she came upon a cat with a half-eaten trout. We have found the remains. There is a triangular gash corresponding with the triangular flap of skin torn off by the sharp stone, and as if justice of nemesis or somebody had assuaged the cat’s appetite at the crucial moment, there is also a shred of skin bearing the unmistakable mark of part of a heel and the scar of a spike.’

‘But can all this …’ Mark began. ‘I mean, when you talk of correspondence –’

‘Our case,’ Alleyn said, ‘will, I assure you, rest upon scientific evidence of an unusually precise character. At the moment I’m giving you the sequence of events. The Colonel’s trout was bestowed upon the cat. Lady Lacklander’s paint rag was used to clean the spike of the shooting-stick and the murderer’s hands. You may remember, Dr Lacklander, that your grandmother said she had put all her painting gear tidily away but you, on the contrary, said you found the rag caught up in a briar bush.’